Exhaust gas purifying apparatus for an internal combustion engine

ABSTRACT

An exhaust purifying apparatus purifies an unburnt gas component, such as unburnt hydrocarbon (HC), discharged from an internal combustion engine without fail and prevents the unburnt hydrocarbon from being discharged into the atmosphere. In order to achieve this, the exhaust gas purifying apparatus is provided with a plurality of exhaust passages connected to the internal combustion engine. A joint exhaust passage is formed by merging the exhaust passages and an exhaust gas flows through the joint exhaust passage. An adsorption/desorption unit is provided in each of the exhaust passages for adsorbing an unburnt gas component contained in the exhaust gas that flows through each of the exhaust passages at a temperature lower than a predetermined temperature. The adsorption/desorption unit desorbs the adsorbed unburnt gas component at a temperature equal to or higher than the predetermined temperature. A desorption/adjustment mechanism synchronizes timing of the unburnt gas component desorbed from the adsorption/desorption units into the exhaust purifying units.

The entire disclosure of Japanese Patent Application No. Hei 10-074690(074690/1998) filed on Mar. 23, 1998 including its specification,claims, drawings, and summary are incorporated herein by reference inits entirely.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to exhaust gas purifying apparatus forpurifying exhaust gas discharged from an internal combustion engine.

2. Description of the Related Art

In an internal combustion engine mounted on an automotive vehicle or thelike, it is necessary to purify discharged exhaust gas, such ascomponents of, for example, carbon monoxide (CO), nitrogen oxide (NOx),or hydrocarbon (HC) before being discharged into the atmosphere.

In particular, it is important to purify the unburnt gas componentsdischarged starting of the internal combustion engine. In this case,when starting the internal combustion engine, an air/fuel ratio ofmixture is set at a low air/fuel ratio (on an enriched side) incomparison with a stoichiometric air/fuel ratio in order to enhancestartability of the internal combustion engine. However, since thetemperature of the internal combustion engine is low and the combustionis unstable, a large amount of the unburnt gas components, such as theunburnt hydrocarbon, is discharged.

To meet this demand, an "engine exhaust gas purifying apparatus"described in Japanese Patent Application Laid-Open No. Hei 6-33747 iswell known. In this apparatus, an adsorbent for adsorbing unburnthydrocarbon (HC) contained in the exhaust gas below a predeterminedtemperature and for releasing the adsorbed unburnt hydrocarbon (HC) at atemperature equal to or higher than the predetermined temperature, isprovided at an exhaust passage upstream of a three way catalyst, and anelectric heated catalyst (EHC) is provided in the exhaust passagebetween the adsorbent and the three way catalyst.

In such an exhaust gas purifying apparatus, in the case where the engineis started in a cold condition so that the three way catalyst is keptunder an inactive condition, the unburnt hydrocarbon (HC) discharged instarting the engine is adsorbed onto the adsorbent and current is fed toa heater of the electric heated catalyst to thereby activate thecatalyst.

Thereafter, when the adsorbent is subjected to heat of the exhaust gasand reaches a predetermined temperature, the unburnt hydrocarbon (HC)adsorbed on the adsorbent is released away from the adsorbent and iscaused to flow into the electric heated catalyst. However, at this time,since the electric heated catalyst is activated by the heater, the abovedescribed unburnt hydrocarbon (HC) is purified by the electric heatedcatalyst.

However, in the case where the above described exhaust gas purifyingapparatus is applied to an internal combustion engine having a pluralityof adsorbents which are provided at parallel exhaust passages upstreamof the electric heated catalyst, an unburnt hydrocarbon (HC) is adsorbedon each adsorbent when starting the internal combustion engine. When atemperature of the adsorbents are elevated to a predeterminedtemperature, the unburnt hydrocarbon (HC) adsorbed on each adsorbent iscaused to be introduced into the electric heated catalyst.

In this case, there are differences of position of the adsorbent in theexhaust passages and of temperature of the exhaust gas introduced intothe adsorbents. As a result, the unburnt hydrocarbon is introduced intothe electric heated catalyst from each adsorbent at different times.This causes the unburnt hydrocarbon, which have been desorbed from theadsorbent, to be introduced into the electric heated catalyst for toolong of a period. In other words, since an air/fuel ratio is rich in theelectric heated catalyst for a long time, the temperature of theelectric heated catalyst is dropped below the activated temperature.Accordingly, in order to maintain the activated temperature of theelectric heated catalyst, it is necessary for current to be fed to theheater for a long time. Thus, electric power in a battery is wasted.Therefore, it is necessary to enlarge battery capacity.

In another case where the above described exhaust gas purifyingapparatus is applied to an internal combustion engine having a pluralityof adsorbents which are provided in parallel exhaust passages upstreamof the three way catalyst without a heater, in warming up the internalcombustion engine, a rich air/fuel ratio gas of unburnt hydrocarbon isdischarged from the engine. At the same time, the unburnt hydrocarbon isdesorbed from the adsorbents when a temperature of each adsorbent iselevated to a predetermined temperature. As a result, since a largeamount of the unburnt hydrocarbon is introduced into the three waycatalyst for a long time, the temperature of the three way catalyst isdropped below the activated temperature. Accordingly, there is apossibility that the unburnt hydrocarbon could be discharged into theatmosphere without completely purifying the exhaust gas through thecatalyst.

SUMMARY OF THE INVENTION

In view of the above-described drawbacks, an object of the presentinvention is to provide a technology for purifying, without fail, anunburnt gas component such as unburnt hydrocarbon (HC) discharged froman internal combustion engine, and to prevent the unburnt gas componentfrom being discharged into the atmosphere.

Another object of the present invention is to provide a technology forpreventing the electric heated catalyst from wasting electric power in abattery.

In order to solve the above-described drawbacks, the present inventionincludes an exhaust gas purifying apparatus for an internal combustionengine comprising: a plurality of exhaust passages connected to amulti-cylinder internal combustion engine; a joint exhaust passageformed by merging the exhaust passages; an exhaust gas purifier forpurifying exhaust gas that flows through the joint exhaust passage; anadsorption/desorption unit provided in each of the exhaust passages foradsorbing an unburnt gas component contained in the exhaust gas thatflows through each of the exhaust passages at a temperature lower than apredetermined temperature and for desorbing the adsorbed unburnt gascomponent at a temperature equal to or higher than the predeterminedtemperature; and a desorption/adjustment mechanism for synchronizingtimings for introducing the unburnt gas component into the exhaust gaspurifier by the adsorption/desorbing means.

In such an exhaust gas purifying apparatus for an internal combustionengine, when the exhaust gas purifier is in the non-active condition andthe above-described adsorption/desorption units are kept below thepredetermined temperature, such as when the internal combustion engineis started in a cold condition, the exhaust gas from the above-describedinternal combustion engine is introduced into the respectiveadsorption/desorption units through the exhaust passages and the unburntgas components contained in the exhaust gas are adsorbed to therespective adsorption/desorption units.

Then, the exhaust gas discharged from the respectiveadsorption/desorption units is introduced into the joint exhaust passagethrough the above-described exhaust passage and subsequently introducedinto the exhaust gas purifier. In this case, the exhaust purifier is inthe non-activated condition, and it is impossible to sufficiently purifythe unburnt gas components contained in the exhaust gas. However, sincethe exhaust gas to be introduced into the above-described exhaust gaspurifier is deprived of the unburnt gas components by theabove-described adsorption/desorption units, the unburnt gas componentswould not be discharged into the atmosphere.

Thereafter, the respective adsorption/desorption units receive heat fromthe exhaust gas and are heated to a predetermined temperature to desorbthe adsorbed unburnt gas component. At this time, thedesorption/adjustment mechanism synchronizes a predetermined timing forthe unburnt gas components, which have been desorbed from the respectiveadsorption/desorption units, to introduce the gas components into theexhaust gas purifier.

As a result, since the unburnt gas components are desorbed from theadsorption/desorption units and introduced together into the exhaust gaspurifier at the same period, the unburnt gas components are purified ina short time by the exhaust gas purifier.

Accordingly, according to the present invention, even if the internalcombustion engine has a plurality of adsorbents which are provided inparallel exhaust passages, it is possible to purify, without fail, theunburnt gas components without increasing the performance or enlargingthe exhaust purifier. It is thus possible to prevent the emission frombeing degraded.

In the exhaust gas purifying apparatus for an internal combustion engineas described above, the adsorption/desorption units may include a threeway catalyst. In general, the three way catalyst is formed by a porouscatalyst layer on a carrier surface. Then, since the exhaust gastemperature is low as in the starting operation of the internalcombustion engine, and the unburnt gas component is in the liquefiedstate or low energy state, if the three way catalyst is less than thepredetermined temperature, the unburnt gas component is adhered to aninterior of holes of the catalyst layer in the liquefied state or lowenergy state. After that, the temperature of the three way catalyst iselevated at a predetermined temperature, the unburnt gas component thathas been adhered to the interior of the holes is gasified or high energystate and released away from holes. Thus, the three way catalyst may beused as the adsorption/desorption units for effecting the adsorption andthe desorption of the unburnt gas component. For theadsorption/desorption units, an adsorbent including a zeolite may alsobe used.

For the exhaust gas purifier, a three way catalyst may be used.

The adsorption/desorption units may control timings of each of theadsorption/desorption units to desorb the unburnt gas componentstherefrom. In this case, the timing of introduction of the unburnt gascomponents into the exhaust gas purifier is the same. Thus, all theunburnt gas components which have been desorbed from all of theadsorption/desorption units can be introduced into the exhaust gaspurifier in a short time.

The desorption/adjustment mechanism may control the temperatures of theexhaust gas introduced in the adsorption/desorption units of eachexhaust passage. Since the adsorption/desorption units receive heat ofthe exhaust gas and the temperature thereof is elevated, thetemperatures of the exhaust gas introduced into the respectiveadsorption/desorption units are controlled relative to each other, sothat it is possible to have predetermined time periods for raisingtemperatures of the adsorption/desorption units to reach thepredetermined temperature, respectively.

As a method for thus controlling the temperatures of the exhaust gasintroduced into the respective adsorption/desorption units, it ispossible to exemplify a method for adjusting the distances between theadsorption/desorption units and the internal combustion engine for everyadsorption/desorption units.

In this case, as the distance to the internal combustion engine isshorter, the adsorption/desorption units will be exposed to the highertemperature of the exhaust gas. Accordingly, the time needed to heat itto the predetermined temperature becomes shorter. And, as the distanceis longer, the time needed to heat it to the predetermined temperaturebecomes longer. Accordingly, the timing of desorption from theadsorption/desorption units is adjusted by the distance.

As a method for controlling the temperatures of the exhaust gasintroduced into the respective adsorption/desorption units, it ispossible to exemplify a method for adjusting the ignition timings of thecylinders to which the respective exhaust passages are connected tothereby control the temperatures of the exhaust gas to be introducedinto the respective exhaust passages. In this case, since in thecylinder having the later ignition timing, the combustion is performedat a later stage in comparison with the cylinders having the earliertimings, the temperature of the combustion gas within that cylinder ishigh in the opening of the exhaust valve. Accordingly, the timing ofdesorption from the adsorption/desorption units is adjusted by theignition timings.

As another method for controlling the temperatures of the exhaust gasdischarged from the respective cylinders, it is possible to exemplify amethod for adjusting the valve opening timings of the exhaust valves ofthe cylinders connected to the respective exhaust passages. Since thetemperature of the burnt gas in each cylinder is lowered when timelapses, the cylinder whose exhaust valve is opened in an earlier stagedischarges the combustion gas kept at a higher temperature than that ofthe cylinder whose exhaust valve is opened in the later stage. Namely,the exhaust gas having a higher temperature than that of the exhaustpassage connected to the cylinder whose exhaust valve is opened in thelater stage is caused to flow through the exhaust passage connected tothe cylinder whose exhaust valve is opened in the earlier stage.

As another method for controlling the temperatures of the exhaust gasdischarged from the respective cylinders, it is possible to exemplify amethod for adjusting the combustion speeds by mixture of the respectivecylinders. The lower the combustion speed, the higher the temperature ofthe combustion gas in the valve opening of the exhaust valve willbecome. Accordingly, the cylinder having lower combustion speeddischarges the combustion gas having the higher temperature than thecylinder having higher combustion speed. Namely, the higher temperatureexhaust gas than the exhaust passage connected to the cylinder havingthe high combustion speed is caused to flow through the exhaust passageconnected to the low combustion speed cylinder.

As another method for controlling the temperatures of the exhaust gasdischarged from the respective cylinders, it is possible to exemplify amethod for adjusting the air/fuel mixture ratios of the respectivecylinders. The higher the air/fuel mixture ratios, the higher thecombustion temperature will become. Accordingly, the higher temperaturecombustion gas is discharged from the cylinder in which the highair/fuel mixture ratio (lean atmosphere mixture) is burnt than thecylinder in which the low air/fuel mixture ratio (enriched atmospheremixture) is burnt. Namely, the higher temperature exhaust gas is causedto flow through the exhaust passage connected to the cylinder in whichthe lean atmosphere mixture is burnt rather than the exhaust passageconnected to the cylinder in which the rich atmosphere mixture is burnt.

As a method for controlling the time needed to elevate the temperatureof the respective adsorption/desorption units to a predeterminedtemperature, it is possible to exemplify a method for adjusting the airamounts drawn in to the cylinders connected to the respective exhaustpassages. Since a large amount of the exhaust gas is discharged from acylinder having a large intake air amount in comparison with a cylinderhaving a small intake air amount, a larger amount of the exhaust gas iscaused to flow through the exhaust passage connected to the cylinderhaving the large intake air amount than the exhaust passage connected tothe cylinder having the small intake air amount.

As a result, the adsorption/desorption unit of the exhaust passageconnected to the cylinder having the large intake air amount is exposedto the larger amount of the exhaust gas than the adsorption/desorptionunit of the exhaust passage connected to the cylinder having the smallintake air amount and reaches the predetermined temperature earlier.

As a method for controlling the temperatures of the exhaust gasintroduced into the respective adsorption/desorption units, it ispossible to exemplify a method for adjusting heat capacities of therespective exhaust passages. Since the exhaust passage having a largerheat capacity adsorbs a larger amount of the heat than the exhaustpassage having a small heat capacity, the exhaust gas flowing throughthe exhaust passage having the large heat capacity is more deprived ofthe heat than the exhaust gas flowing through the exhaust passage havingthe small heat capacity and the temperature when the exhaust gas isintroduced into the adsorption/desorption unit becomes low.

In the case where the exhaust gas temperatures introduced into therespective adsorption/desorption units are adjusted relative to eachother by the above-described method, it is possible to adjust thepredetermined time period for raising temperatures of theadsorption/desorption units to reach the predetermined temperature,respectively. Even in the case where adsorption/desorption units havingthe same function are used, it is possible to synchronize the time whenthe unburnt gas component is introduced into the exhaust gas purifier.

As a method for controlling the desorption timings of the respectiveadsorption/desorption units, it is possible to exemplify a method foradjusting the heat capacities of the respective adsorption/desorptionunits. An adsorption/desorption unit having a larger heat capacity has alarger amount of heat to be adsorbed in comparison with aadsorption/desorption unit having a smaller heat capacity and it takes alonger time to elevate the temperature to a predetermined temperature.

In the case where each adsorption/desorption unit is provided with acarrier having a plurality of through holes in a direction of the flowof the exhaust gas, a catalyst layer formed on a surface of the carrierand an outer sleeve incorporating therein the carrier, it is possible toadjust at least one factor selected from a thickness of a memberconstituting the carrier, a thickness of a member constituting the outersleeve, a density of the through holes, a diameter of the carrier, anaxial length of the carrier and a volume of the carrier for everyadsorption/desorption unit as a method for adjusting heat capacities.

For example, in case of the two adsorption/desorption units having thesame structure except for the carrier thickness, theadsorption/desorption unit having the larger thickness is able to adsorba larger amount of heat than the adsorption/desorption unit having thesmaller thickness. As a result, it takes a longer time to raise thetemperature of the adsorption/desorption unit having the larger carrierthickness to reach the predetermined temperature in comparison with theadsorption/desorption unit having the smaller carrier thickness.

In case of the two adsorption/desorption units having the same structureexcept for outer sleeve thickness, the adsorption/desorption unit havingthe larger outer sleeve thickness is able to adsorb a larger amount ofheat than the adsorption/desorption unit having the smaller outer sleevethickness. As a result, it takes a longer time to raise the temperatureof the adsorption/desorption unit having the larger thickness to reachthe predetermined temperature in comparison with theadsorption/desorption unit having the smaller thickness.

In case of the two adsorption/desorption units having the same structureexcept for the density of the through holes, i.e., the twoadsorption/desorption units have the same structure except for thenumber of the through holes per units area of the carrier, theadsorption/desorption unit provided with the higher density of throughholes is able to adsorb a larger amount of heat than theadsorption/desorption unit provided with the lower density of throughholes. As a result, it takes a longer time to raise the temperature ofthe adsorption/desorption unit having the higher density to reach thepredetermined temperature in comparison with the adsorption/desorptionunit having lower density.

In the case where a diameter of the carrier of the adsorption/desorptionunits is adjusted, i.e., when two adsorption/desorption units have thesame structure other than the diameter of the carrier, theadsorption/desorption unit having a larger carrier diameter has a largersubstantial volume of the carrier in comparison with theadsorption/desorption unit having a smaller diameter of the carrier andmay have a larger amount of heat to be adsorbed. As a result, it takes alonger time to raise the temperature of the adsorption/desorption unithaving the larger carrier diameter to reach the predeterminedtemperature in comparison with the adsorption/desorption unit having thesmaller carrier diameter.

In the case where an axial length of the carrier of theadsorption/desorption units is adjusted, i.e., when twoadsorption/desorption units have the same structure other than the axiallength of the carrier, the adsorption/desorption unit having a longeraxial length of the carrier has a larger substantial volume of thecarrier in comparison with the adsorption/desorption unit having ashorter axial length of the carrier and may have a larger amount of heatto be adsorbed. As a result, for the adsorption/desorption unit havingthe longer axial length of the carrier, it takes a longer time until theheat is conducted to the end portion of the outlet of theadsorption/desorption unit.

Therefore, it takes a longer time to raise the temperature of theadsorption/desorption unit having the longer axial length of the carrierto reach the predetermined temperature in comparison with theadsorption/desorption unit having the shorter axial length of thecarrier.

In the case where the volume of the carrier is adjusted, theadsorption/desorption unit having a larger volume of the carrier has alarger amount of heat to be adsorbed than the adsorption/desorption unithaving a smaller volume of the carrier. As a result, it takes a longertime to raise the temperature of the adsorption/desorption unit having alarger volume of the carrier to reach the predetermined temperature incomparison with the adsorption/desorption unit having a smaller volumeof the carrier.

As another method for adjusting the heat capacities of theadsorption/desorption units, it is possible to adjust at least one ofthe material of the member constituting the carrier, an amount of thecatalyst substance and an amount of the catalyst layer for everyadsorption/desorption unit.

For example, where the material constituting the carrier is a materialhaving a large heat capacity and a material having a small heatcapacity, the adsorption/desorption unit having the carrier made of thelarge heat capacity material is able to adsorb a larger amount of heatthan the adsorption/desorption unit having the carrier made of the smallheat capacity material. As a result, it takes longer time to raise thetemperature of the adsorption/desorption unit having the carrier made ofthe large heat capacity material to reach the predetermined temperaturein comparison with the adsorption/desorption unit having the carriermade of the small heat capacity material.

In the case where the amount of the catalyst material carried on thecarrier is adjusted, the adsorption/desorption unit having the largeramount of catalyst substance on the carrier has a substantially largerheat capacity than the adsorption/desorption unit having the smalleramount of catalyst substance on the carrier.

Where the amount of the catalyst layer is adjusted, theadsorption/desorption unit having the larger amount of the catalystlayer has a substantially larger heat capacity than theadsorption/desorption unit having the smaller amount of the catalystlayer.

Where the heat capacity is adjusted for the respectiveadsorption/desorption units by the above-described methods, it ispossible to control the desorption timings of the unburnt gas componentin the respective adsorption/desorption units without complicating thecontrol of the internal combustion engine.

Here the above-described exhaust passage may be a dual exhaust pipeconnected to the internal combustion engine and may be exhaust pipesconnected to each cylinder bank in the case of a V-shaped internalcombustion engine provided with a first cylinder bank and a secondcylinder bank having at least two cylinders arranged in a line.

For the exhaust gas purifier, a electric heated catalyst may be used. Acurrent is fed to the electric heated catalyst prior to when the unburntgas component from the respective adsorption/desorption units isintroduced into the electric heated catalyst. As a result, since theelectric heated catalyst is activated when the unburnt gas component isintroduced into the electric heated catalyst, the unburnt gas componentis purified in a short time and efficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to the accompanying drawings, wherein:

FIG. 1 is a schematic view showing a structure of an internal combustionengine and an exhaust system to which an exhaust gas purifying apparatusaccording to the present invention is applied;

FIG. 2 is a view showing a structure of a first three way catalyst;

FIG. 3 is a view showing a structure of a catalyst;

FIG. 4 is a view showing an adsorption performance of the first threeway catalyst;

FIG. 5 is a view showing an unburnt hydrocarbon desorption timing of thefirst three way catalyst and a second three way catalyst;

FIG. 6 is a schematic view showing a structure of an internal combustionengine and an exhaust system to which an exhaust gas purifying apparatusaccording to a second embodiment is applied;

FIG. 7 is a view showing a relationship between an ignition timing of afirst cylinder bank and an ignition timing of a second cylinder bank;

FIG. 8 is a view showing a fuel injection timing of the first cylinderbank and a fuel injection timing of the second cylinder bank;

FIG. 9 is a view showing a relationship between an air/fuel ratio of thefirst cylinder bank and an air/fuel ratio of the second cylinder bank;

FIG. 10 is a schematic view showing a structure of an internalcombustion engine and an exhaust system to which an exhaust gaspurifying apparatus according to a fourth embodiment is applied;

FIG. 11 is a view showing a relationship between an exhaust gas amountof the first cylinder bank and an exhaust gas amount of the secondcylinder bank;

FIG. 12 is a schematic view showing a structure of an internalcombustion engine and an exhaust system to which an exhaust gaspurifying apparatus according to a fifth embodiment is applied;

FIG. 13 is a schematic view showing a structure of an internalcombustion engine and an exhaust system to which an exhaust gaspurifying apparatus according to a sixth embodiment is applied;

FIG. 14 is a schematic view showing a structure of an internalcombustion engine and an exhaust system to which an exhaust gaspurifying apparatus according to a seventh embodiment is applied;

FIG. 15 is a view showing a relationship between a heat capacity of thethree way catalyst and a desorption timing of the unburnt hydrocarbon;and

FIG. 16 is a view showing another embodiment of an internal combustionengine to which an exhaust gas purifying apparatus according to thepresent invention is applied.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a first embodiment of the invention will now bedescribed with reference to the accompanying drawings.

FIG. 1 is a schematic structural view showing a structure of an internalcombustion engine to which an exhaust gas purifying apparatus for aninternal combustion engine in accordance with the invention is applied,and its structure of the exhaust gas system. An arrow "F" in the FIG. 1shows the front direction of the internal combustion engine.

The above-described internal combustion engine 1 is a v-shaped enginehaving six cylinders and arranged lengthwise. A first exhaust manifold 2is connected to a bank of cylinders 1a on one side (hereinafter referredto as a first cylinder bank 1a) and a second exhaust manifold 3 isconnected to a bank of cylinders 1b on the other side (hereinafterreferred to as a second cylinder bank 1b).

Then, the first exhaust manifold 2 is connected to a first exhaust pipe4 used as a first exhaust passage according to the present invention,and the second exhaust manifold 3 is connected to a second exhaust pipe5 used as a second exhaust passage according to the present invention.

Here, the first exhaust pipe 4 and the second exhaust gas pipe 5 take asubstantially symmetrical arrangement, and are structured so that adistance from a joint portion 16 to the first cylinder bank 1a of thefirst exhaust manifold 2 and the first exhaust pipe 4 is equal to thatfrom a joint portion 17 to the second cylinder bank 1b of the secondexhaust manifold 3 and the second exhaust pipe 5.

Subsequently, the first exhaust pipe 4 and the second exhaust pipe 5 aremerged together on the downstream side and connected to an exhaust pipe6 as a common exhaust passage according to the present invention. A fistthree way catalyst 7 is disposed midway along the first exhaust pipe 4and a second three way catalyst 8 is disposed midway along the secondexhaust pipe 5. A distance X from the joint portion 16 to an inletportion of the first three way catalyst 7 of the first exhaust manifold2 and the first exhaust pipe 4 is equal to distance Y from the jointportion 17 to an inlet portion of the second three way catalyst 8 of thesecond exhaust manifold 3 and the second exhaust pipe 5.

Then, as shown in FIG. 2, the first three way catalyst 7 is formed byfilling a cylindrical outer sleeve 7a with a monolithic type catalyst 7bhaving a plurality of through-holes in the flow direction of the exhaustgas. More specifically, as shown in FIG. 3, the catalyst 7b is composedof a ceramic carrier 7c made of corgelite formed into a lattice so as tohave the through-holes in the flow direction of the exhaust gas and acatalyst layer 7d coated on a surface of the ceramic carrier 7c.

As shown in FIG. 4, the above-described catalyst layer 7d is formed bycarrying a platinum-rhodium (Pt--Rh) system noble metal catalystsubstance 7e on a surface of porous alumina (Al₂ O₃) having a pluralityof pores 7f.

In the thus formed first three way catalyst 7, when a temperature of thecatalyst 7b is lower than a predetermined temperature, an unburnt gascomponent in a liquid form such as hydrocarbon (HC) included in theexhaust gas flows into the pores 7f of the catalyst 7d and adhered towall surfaces of the pores 7f. Then, when the temperature of thecatalyst 7b reaches the predetermined temperature or more, the unburnthydrocarbon adhered within the above-described pores 7f is gasified andreleased from the above-described catalyst layer 7d to flow on thedownstream side together with the exhaust gas. Namely, the first threeway catalyst 7 realizes an adsorption/desorption units according to thepresent invention.

Also, the above-described second three way catalyst 8 is formed in thesame way as that for the first three way catalyst 7 and realizes theadsorption/desorption units according to the present invention.

Turning back to FIG. 1, a third three way catalyst 9 incorporating aheater 12 for heating by electric application is provided midway alongthe exhaust pipe 6. The above-described heater 12 is connected through arelay 13 to a battery 14 and generates heat by the current from thebattery 14 when the relay 13 is turned on. The ON/OFF condition of theabove-described relay 13 is switched in accordance with an electricsignal from an ECU (electronic control units) 15.

Subsequently, a secondary air feeding pipe 10 is connected to theexhaust pipe 6 upstream of the third catalyst 9 and is connected to anair pump 11. The air pump 11 is driven in accordance with an electricsignal from the ECU 15 for pressurizing and feeding fresh air, flowingthrough an intake passage downstream of the air cleaner (not shown), tothe exhaust pipe 6.

Air/fuel ratio sensors 25 and 26 are mounted on the first exhaust pipe 4upstream of the first three way catalyst 7 and on the second exhaustpipe 5 upstream of the second three way catalyst 8, respectively. Eachof these air/fuel ratio sensors 25 and 26 includes a solid electrolyteportion formed into a cylinder by sintering zirconia (ZrO₂), an outerplatinum electrode covering an outer surface of the solid electrolyteportion, and an inner platinum electrode covering an inner surface ofthe solid electrolyte portion. The sensor is a so-called linear air/fuelsensor, which outputs a current in proportion to an oxygen concentrationof the exhaust gas (concentration of the unburnt gas component when theair/fuel ratio is more on the enrich side) in accordance with the oxygenion movement when the voltage is applied between the above-describedelectrodes.

An oxygen sensor 24 for detecting an oxygen concentration of the exhaustgas flowing through the exhaust pipe 6 is mounted on the exhaust pipe 6downstream of the third three way catalyst 9. For instance, the oxygensensor 24 is a zirconia type sensor which outputs an electromotive forcethat exceeds a predetermined level in an enriched atmosphere withrespect to a stoichiometric air/fuel ratio and outputs an electromotiveforce that is less than the predetermined level in a lean atmosphere.

Subsequently, the ECU 15 for controlling the respective above-describedportions is connected to various sensors (not shown) in addition to anignition switch sensor (hereinafter referred to as a IG sensor) 22, astarter switch sensor (hereinafter referred to as a ST sensor) 23, theair/fuel ratio sensors 25 and 26 and the oxygen sensors 24, an air flowsensor (not shown), an engine rotational speed sensor (not shown), anengine coolant temperature sensor (not shown), a catalyst temperaturesensor (not shown) or the like. ECU 15 calculates an electricapplication timing of the heater 12, a secondary air feeding amount, asecondary air feeding timing, a fuel injection amount (a length of thefuel injection time), a fuel injection timing, an ignition timing or thelike, to control the relay 13, the air pump 11, fuel injectors (notshown), an ignition system (not shown) and the like in accordance withthe signals from the respective sensors.

For instance, the ECU 15 starts the electric application to the heater12 when an electric signal representative of the ON condition of theignition switch is fed to the ECU 15 by the IG sensor 22. Then, the ECU15 calculates the current application period for the heater inaccordance with a heater map showing a relationship of an engine coolanttemperature and the current application period at the start of theinternal combustion engine 1.

Also, the ECU 15 feeds a drive current to the air pump 11 when anelectric signal representative of the ON switch of the starter switch isapplied to the ECU 15 by the ST sensor 23.

Subsequently, the ECU 15 performs a so-called air/fuel ratio feedbackcontrol which compensates for the fuel amount injected into therespective intake ports or the respective cylinders of the firstcylinder bank la and the second cylinder bank 1b in response to theelectric signals from the respective air/fuel ratio sensors 25 and 26and simulates the exhaust gas discharged from the first cylinder bank 1aand the second cylinder bank 1b to an air/fuel ratio at which the firstthree way catalyst 7 and the second three way catalyst 8 effectivelywork.

Furthermore, the ECU 15 detects the oxygen concentration downstream ofthe third three way catalysts 9 by the oxygen sensors 24 and compensatesfor the control amount of the air/fuel ratio feedback control by theair/fuel ratio sensors 25 and 26 so that the air/fuel ratio of theexhaust gas introduced into the first, second and third three waycatalysts 7, 8 and 9 respectively maintains the stoichiometric air/fuelratio.

The operation and advantage of the exhaust gas purifying apparatus foran internal combustion engine in accordance with the embodiment will nowbe described.

When the electric signal representative of the ON condition of theignition switch is inputted into the ECU 15 by the IG sensor 22 , theECU 15 reads the current application period and the current applicationON timing from the heater map. Then, the ECU 15 switches the relay 13from the OFF condition to the ON condition and applies the current fromthe battery 14 to the heater 12 of the third three way catalyst 9.

It should be noted that, in the case where the exhaust gas having theair/fuel ratio close to the stoichiometric air/fuel ratio is introducedinto the three way catalyst, the hydrocarbon HC and the carbonmonoxideCO contained in the exhaust gas are reacted with the oxygen O₂ and areoxidized into H₂ O and CO₂, and simultaneously therewith, the NOxcontained in the exhaust gas is reduced into H₂ O, CO₂ and N₂. However,as in the case of the start of the internal combustion engine when theincreased compensation of the fuel injection amount is effected and theoxygen concentration of the exhaust gas is low and the HC and the CO areexcessive, the NOx contained in the exhaust gas is reacted with the HCand CO and reduced into H₂ O, CO₂ and N₂ but the excessive HC and CO arenot oxidized.

Therefore, the ECU 15 feeds a drive current to the air pump 11 when anelectric signal representative of the on switch of the starter switch isapplied to the ECU 15 by the ST sensor 23. The ECU 15 pressurizes andfeeds the fresh air that flows through the intake passage downstream ofthe air cleaner. At this time, the secondary air is fed into the exhaustgas flowing through the exhaust pipe 6 so that the air/fuel ratio of theexhaust gas which is introduced into the third three way catalyst 9 ischanged on the lean side.

Subsequently, when the start of the internal combustion engine 1 iscompleted, the exhaust gas from the respective cylinders of the firstcylinder bank la of the internal combustion engine 1 is introduced intothe first exhaust pipe 4 through the first exhaust manifold 2, and theexhaust gas from the respective cylinders of the second cylinder bank 1bare introduced into the second exhaust pipe 5 through the second exhaustmanifold 3.

The exhaust gas which has been introduced into the first exhaust pipe 4is introduced into the first three way catalyst 7 in the first exhaustpipe 4 and the unburnt hydrocarbon (HC) contained in the exhaust gas istemporarily adsorbed onto the first three way catalyst 7. Subsequently,the unburnt gas component discharged from the first three way catalyst 7is introduced into the third three way catalyst 9 through the firstexhaust pipe 4 and the exhaust pie 6.

On the other hand, the exhaust gas which has been introduced into thesecond exhaust pipe 5 is introduced into the second three way catalyst 8in the second exhaust pipe 5 and the unburnt hydrocarbon (HC) containedin the exhaust gas is temporarily adsorbed onto the second three waycatalyst 8. Then, the exhaust discharged from the second three waycatalyst 8 is introduced into the third three way catalyst 9 through thesecond exhaust pipe 5 and the exhaust pipe 6.

The unburnt hydrocarbon (HC) of the exhaust gas from the first exhaustpipe 4 and the second exhaust pipe 5 is removed by the first three waycatalyst 7 and the second three way catalyst 8. Accordingly, even if thethird three way catalyst 9 is not active, the unburnt hydrocarbon (HC)is not discharged downstream of the third three way catalyst 9.

Subsequently, the temperature of the first three way catalyst 7 and thesecond three way catalyst 8 is elevated earlier than the third three waycatalyst 9 by the heat of the exhaust gas. Since the distance from thefirst three way catalyst 7 to the exhaust port of the engine is equal tothe distance from the second three way catalyst 8 to the exhaust port ofthe engine, the temperature elevation rate of the first three waycatalyst 7 is equal to that of the second three way catalyst 8. As aresult, since the desorption timing of the first three way catalyst 7 isequal to that of the second three way catalyst 8, the unburnthydrocarbon (HC) which is desorbed from the first and second catalysts 7and 8 is introduced into the third catalyst 9 at the same timing. Atthis time, since the third catalyst 9 is activated by the heater 12, theunburnt hydrocarbon (HC) is purified in a short time and efficiently.Since the current application period of heater 12 is short, theconsumption of the electric power in a battery is able to be reduced.

Here, the desorption timing of the first three way catalyst 7 and thedesorption timing of the second three way catalyst 8 will now bedescribed with reference to FIG. 5. The horizontal axis in FIG. 5 showsthe passing time from the start of the internal combustion engine, thevertical axis in FIG. 5 shows the concentration of an unburnthydrocarbon (HC). A curve "a" in FIG. 5 is a curve representative of aresult from the measurement of the HC concentration of the exhaust gasupstream of the first three way catalyst 7 or the second three waycatalyst 8 and shows the existence of a large amount of the HC in theexhaust gas in the start of the internal combustion engine 1.

Subsequently, a curve "b" in FIG. 5 is a curve representative of aresult from the measurement of the HC concentration of the exhaust gasdownstream of the first three way catalyst 7 and shows the fact that theHC concentration is high at the time when about twenty-five seconds havelapsed from the start of the internal combustion engine 1 and the HC hasbeen desorbed from the first three way catalyst 7.

Then, a curve "c" in FIG. 5 is a curve representative of a result fromthe measurement of the HC concentration of the exhaust gas downstream ofthe second three way catalyst 8 and shows the fact that the HCconcentration is high at the time when about twenty-five minutes havelapsed from the start of the internal combustion engine 1 and the HC hasbeen desorbed from the second three way catalyst 8. Accordingly, FIG. 5shows the fact that the unburnt hydrocarbon is desorbed from the firstand second three way catalysts 7 and 8 at the same period.

Thus, it is possible to realize a desorption adjustment means forcontrolling the temperatures of the exhaust gas introduced into thefirst three way catalyst 7 and the second three way catalyst 8 from eachother by equaling both the distance from the first three way catalyst 7to the exhaust port of the internal combustion engine 1 and the distancefrom the second three way catalyst 8 to the exhaust port of the internalcombustion engine 1.

Then, since the temperature elevation rate of the first three waycatalyst 7 is equal to that of the second three way catalyst 8, theunburnt hydrocarbon is desorbed from the first and second three waycatalysts 7 and 8 at the same period. And, the unburnt hydrocarbon whichis desorbed from the first and second three way catalysts 7 and 8introduces into the third three way catalyst 9 at the same time.

As a result, by the exhaust gas purifying apparatus for an internalcombustion engine according to this embodiment, since the time when theunburnt hydrocarbon is introduced into the third three way catalyst 9 isshort, it is possible to prevent the catalyst temperature of the thirdthree way catalyst 9 from dropping below the activated temperature.Accordingly, it is possible to purify all the unburnt hydrocarbon (HC)which has been desorbed from the first three way catalyst 7 and thesecond three way catalyst 8. And, it is possible to suppress thecapacity increase of the third three way catalyst and the enlargement ofthe heater 12 and battery 14 by the capacity increase and to prevent theelectric power from being wasted. An exhaust gas purifying apparatus foran internal combustion engine in accordance with a second embodiment ofthe present invention will now be described with reference to thedrawings. In this case, only a structure which is different from thefirst embodiment will be described.

FIG. 6 is a view showing a schematic structure of an internal combustionengine 1 to which the exhaust gas purifying apparatus for an internalcombustion engine in accordance with this embodiment is applied and anexhaust system thereof. An arrow "F" in the FIG. 6 shows the frontdirection of the internal combustion engine.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b. As an example, there is a V-shaped internalcombustion engine arranged sideways in an engine room. Namely, in caseof the engine having the first cylinder bank 1a on the front side andthe second cylinder 1b on the back side, since there are various engineparts in the engine room, the distance from the first three way catalyst7 to the exhaust port is different from the distance from the secondthree way catalyst 8 to the exhaust port.

As a result, the temperature of exhaust gas which is introduced into thefirst three way catalyst 7 is different from the temperature of exhaustgas which is introduced into the second three way catalyst 8, and, thetiming when the unburnt hydrocarbon HC is desorbed from the first threeway catalyst 7 is different from the timing when the unburnt hydrocarbonHC is desorbed from the second three way catalyst 8. But, in the secondembodiment, the temperature of the exhaust gas which is introduced intoeach three way catalyst is equally controlled by adjusting the ignitiontiming in each cylinder of the engine.

Then, an ignition coil 18 is provided in each cylinder of the internalcombustion engine 1 for converting a low voltage current to a highvoltage current from an ignitor 19 and for applying it to each ignitionplug. The ignitor 19 applies a low voltage drive current to eachignition coil 18 in accordance with a control signal from the ECU 15.

Also, the internal combustion engine 1 is provided with a crank anglesensor 21 for outputting an electric signal every 10° of the crankshaft(not shown) and a water temperature sensor 28 for detecting atemperature of cooling water.

Furthermore, cam position sensors 20 are mounted on the cylinder headsof the respective cylinder banks 1a and 1b of the internal combustionengine 1 for detecting rotational positions of cam shafts (not shown).An air flow meter 30 for outputting an electric signal in correspondencewith an air mass flowing through the intake pipe (not shown) of theinternal combustion engine 1 is mounted in the intake pipe.

The above-described cam position sensors 20 are electromagneticpickup-type sensors for outputting electric signals before the top deadcenter of the compression stroke of the cylinder which is a referencecylinder. In this case, the above-described cam position sensors 20 areset so that the electric signal outputted from the crank angle sensor 21immediately after the output of the cam position sensors 20 is set at10° before the top dead center of the compression stroke of theabove-described reference cylinder.

Subsequently, the ECU 15 is connected to various sensors (not shown) inaddition to an IG sensor 22, a ST sensor 23, the air/fuel sensors 25 and26, the oxygen sensor 24, the cam position sensors 20, theabove-described crank angle sensor 21, the water temperature sensor 28and calculates an electric application ON timing and period of theheater 12, a secondary air feeding amount, a secondary air feedingtiming, a fuel injection amount(a length of the fuel injection time), afuel injection timing, an ignition timing or the like, to control therelay 13, the air pump 11, the ignitor 19 and the like in accordancewith the signals from the respective sensors. For instance, when theignition timing of each cylinder of the internal combustion engine 1 isdetermined, the ECU 15 uses the 10° before the above-describedcompression top dead center as an ignition reference position of theabove-described reference cylinder and compensates for theabove-described ignition reference position in accordance with thecooling water temperature, the engine RPM or the intake pipe vacuumpressure to thereby calculate the optimum ignition timing.

Then, in the starting operation of the internal combustion engine 1, theECU 15 seeks the ignition reference position of the above-describedreference cylinder in accordance with a signal from the above-describedcam position sensor 20 and the crank angle sensor 21. Subsequently,after the completion of the starting operation of the internalcombustion engine 1, the ECU 15 accumulates a basic advance angle on thebasis of the intake pipe vacuum pressure, the engine RPM or the like,and simultaneously calculates the warming-up compensation advance angleon the basis of the cooling water temperature to determine the ignitiontiming of each cylinder by adding the above-described basic advanceangle and the above-described warming-up compensation advance angle tothe above-described reference position.

Furthermore, in accordance with this embodiment, in the warming-upoperation after the completion of the starting operation of the internalcombustion engine 1, the ignition timing of the second cylinder bank 1bis delayed to the ignition timing of the first cylinder bank la so thatthe temperature change of the exhaust gas which is caused by thedifference of the distance from each three way catalyst to the exhaustport is corrected. As a result, the temperature of the exhaust gasintroduced into each three way catalyst changes equally. In this case,the ECU 15 sets the warming-up timing in response to the temperature ofthe cooling water for the starting operation and determines the ignitiontimings of each cylinder bank 1a and 1b in accordance with a map asshown in FIG. 7.

In FIG. 7, the ignition timing of the first cylinder bank 1a and thesecond cylinder bank 1b to the above-described ignition referenceposition (10° before the compression top dead center of each cylinder)in the starting operation of the internal combustion engine 1.

Then, in the warming-up operation after the completion of the startingoperation of the internal combustion engine 1, the ignition timing ("d"in FIG. 7) of the first cylinder bank 1a is set at about 5° immediatelybefore the compression top dead center and simultaneously the ignitiontiming ("e" in FIG. 7) of the second cylinder bank 1b is set at a delayto the ignition timing of the above-described first cylinder bank 1a (inthe vicinity of the compression top dead center) The operation andadvantage of the exhaust gas purifying apparatus for an internalcombustion engine in accordance with this embodiment will now bedescribed.

When the internal combustion engine 1 is to be started, the ECU 15judges the start of the internal combustion engine 1 at the moment whenthe signal from the IG sensor 23 is received, and receives the electricsignals from the crank angle sensor 21, the cam position sensors 20, thewater temperature sensor 28 and the air flow meter 30.

Then, the ECU 15 calculates the ignition timing (valve opening period)of each fuel injection valve in accordance with the signal from eachsensor, judges the ignition reference position of the reference cylinderin accordance with the electric signals from the cam position sensor 20and the crank angle sensor 21 and feeds the ignition signal to theignitor 19 while regarding the above-described ignition referenceposition as the ignition timing of the above-described referencecylinder.

When the ignitor 19 receives the ignition signal, the ignitor 19 appliesa low voltage drive current to the ignition coil 18 of theabove-described reference cylinder. At this time, the ignition coil 18of the above-described reference cylinder converts the above-describeddrive current to a high voltage drive current and applies it to theignition plug. Subsequently, after the ECU 15 outputs theabove-described ignition signal, the ECU 15 calculates the ignitiontiming for the next cylinder when the first electric signal is receivedfrom the crank angle sensor 21.

When the internal combustion engine 1 is started by repeating suchoperation, as described in conjunction with the above-described firstembodiment, a large amount of unburnt hydrocarbon (HC) is dischargedfrom the internal combustion engine. The unburnt hydrocarbon (HC) istemporarily adsorbed by the first three way catalyst 7 and the secondthree way catalyst 8.

Then, as mentioned above in conjunction with FIG. 7, during thewarming-up operation of the internal combustion engine 11 the ECU 15determines the ignition timing for each cylinder so that the ignitiontiming of the second cylinder bank 1b is delayed to the ignition timingof the first cylinder bank 1a. In this case, since the conditions otherthan that for the ignition timing are set at the same conditions for thefirst cylinder bank 1a and the second cylinder bank 1b, the combustionof each cylinder of the second cylinder bank 1b is performed at thetiming delayed from each cylinder of the first cylinder bank 1a. At thevalve opening timing of the exhaust valve (not shown), the combustiongas temperature of each cylinder of the second cylinder bank 1b ishigher than the combustion gas temperature of each cylinder of the firstcylinder bank 1a.

As a result, the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is higher than that of the exhaust gasdischarged from the first cylinder bank 1a. However, since the secondthree way catalyst 8 is provided at a position farther from the exhaustport, the temperature change of the first three way catalyst 7 issubstantially similar to the temperature change of the second three waycatalyst 8.

Namely, even if the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is high, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the ignition timing of eachcylinder.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same time and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromeach three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon introduces into the third three way catalyst 9 is short, itis possible to prevent the catalyst temperature of the third three waycatalyst 9 from dropping below the activated temperature. Accordingly,it is possible to purify all the unburnt hydrocarbon (HC) which has beendesorbed from the first three way catalyst 7 and the second three waycatalyst 8. Also, it is possible to suppress the capacity increase ofthe third three way catalyst 9 and the enlargement of the heater 12 andbattery 14 to prevent electric power from being wasted.

Incidentally, there is described an example in which the distance fromthe first three way catalyst 7 to the exhaust port of the first cylinderbank 1a is shorter than the distance from the second three way catalyst8 to the exhaust port of the second cylinder bank 1b.

However, needless to say, it is possible to set that the distance fromthe second three way catalyst 8 to the exhaust port of the secondcylinder bank 1b shorter than the distance from the first three waycatalyst 7 to the exhaust port of the first cylinder bank 1a.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a third embodiment of the present invention will now bedescribed with reference to the drawings. In this case, only structurethat is different from the first and second embodiment will bedescribed. In this embodiment, as the second embodiment in FIG. 6, thedistance from the first three way catalyst 7 to the exhaust port of thefirst cylinder bank 1a is shorter than the distance from the secondthree way catalyst 8 to the exhaust port of the second cylinder bank 1b.

In FIG. 6, a fuel injection valve 27 is mounted on each intake port oreach cylinder of the internal combustion engine 1. When a drive currentis applied from a drive circuit 29, the fuel injection valve 27 isopened for injection of fuel. The above-described drive circuit 29applies the drive current to each fuel injection valve 27 in accordancewith a control signal from the ECU 15.

For instance, when a length of the fuel injection time of each cylinderof the internal combustion engine 1 is to be determined, the ECU 15calculates the engine RPM in accordance with the electric signal fromthe above-described crank angle sensor 21, calculates the basic fuelinjection amount (the basic length of the fuel injection time for thefuel injection valve corresponding to each cylinder) of each cylinder inaccordance with the electric signals from the water temperature sensor28 and the above-described air flow meter 30 and the engine RPM thuscalculated, and determines the fuel injection timing of each fuelinjection valve 27 by compensating for the calculated basic fuelinjection amount in response to an air/fuel ratio, the intake airtemperature, the cooling water temperature or the operational conditionof the internal combustion engine.

More specifically, the ECU 15 calculates the engine RPM in cranking inaccordance with the electric signal from the above-described crank anglesensor 21 in the starting operation of the internal combustion engine 1and determines the length of the fuel injection time in response to thecalculated engine RPM and the electric signal from the water temperaturesensor 28.

Then, when the starting operation of the internal combustion engine iscompleted, the ECU 15 calculates the basic length of the fuel injectiontime on the basis of the intake air amount and the engine RPM andcompensates for the above-described basic length of the fuel injectiontime in response to the water temperature to determine the length of thefuel injection time.

Furthermore, in accordance with this embodiment, during the warming-upoperation after the completion of the starting operation of the internalcombustion engine 1, the temperature of exhaust gas that is introducedinto each three way catalyst is controlled by adjusting the air/fuelmixture ratio to be burnt in each cylinder. In this case, the ECU 15sets the warming-up time in response to the temperature of the coolingwater in the starting operation and subsequently calculates the lengthof the fuel injection time of each cylinder bank 1a, 1b in accordancewith a map shown in FIG. 8.

A curve "f" in FIG. 8 is a curve representative of the length of thefuel injection time of the first cylinder bank 1a, and a curve "g" is agraph representative of the length of the fuel injection time of thesecond cylinder bank 1b. In this case, during the starting operation ofthe internal combustion engine 1, lengths of fuel injection times of thefirst cylinder bank 1a and the second cylinder bank 1b are set in thesame manner and in order to enhance the startability of the internalcombustion engine 1, the mixture of the atmosphere is enriched.

Subsequently, when the starting operation of the internal combustionengine 1 is completed and moved to the warming-up condition, the lengthof the fuel injection time of the first cylinder bank 1a is set to belonger than the length of the fuel injection time of the second cylinderbank 1b. Furthermore, when the vehicle runs, the length of the fuelinjection time of the first cylinder bank 1a and the length of the fuelinjection time of the second cylinder bank 1b are set to be the same.

By thus setting lengths of fuel injection times of the respectivecylinder banks 1a and 1b, as shown in FIG. 9, the air/fuel ratio of eachcylinder bank 1a, 1b is set so that the air/fuel ratio (curve "i" inFIG. 9, for example, air/fuel ratio=12.0) is shifted more on theenriched side atmosphere than the air/fuel ratio (curve "h" in FIG. 9,for example, air/fuel ratio=13.5) of the second cylinder bank 1b. Thecontrol of air/fuel ratio corresponds to the desorption/adjustmentmechanism of the present invention.

The operation and advantage of the exhaust gas purifying apparatus foran internal combustion engine in accordance with this embodiment willnow be described.

When the internal combustion engine 1 is to be started, the ECU 15judges the start of the internal combustion engine 1 at the moment whenthe signal from the ST sensor 23 is received, and receives the electricsignals from the crank angle sensor 21, the water temperature sensor 28and the air flow meter 30. Then, the ECU 15 calculates the length of thefuel injection time (valve opening period) of each fuel injection valvein accordance with the signal from each sensor, and calculates the fuelinjection starting timing of each fuel injection valve in accordancewith the signal from the above-described crank angle sensor 21.

Subsequently, the ECU 15 refers to the signal from the crank anglesensor 21, and feeds a signal representative of the above-describedlength of the fuel injection time to the drive circuit 29 when therotational position of the crankshaft reaches the above-described fuelinjection starting timing.

The drive circuit 29 applies the drive current to the fuel injectionvalve 27 of each cylinder when it receives the signal representative ofthe length of the fuel injection time. Then, the drive circuit 29 stopsthe application of the drive current to the above-described fuelinjection valve 27 at the moment when the above-described length of thefuel injection time has lapsed from the application start of the drivecurrent. At this time, the fuel injection valve 27 of each cylindercontinuously opens during the period of the drive current applicationfrom the drive circuit 29 and continuously injects the fuel.

When the internal combustion engine 1 is started by repeating suchoperation, as described in conjunction with the above-described firstembodiment, a large amount of unburnt hydrocarbon (HC) is dischargedfrom the internal combustion engine. The unburnt hydrocarbon (HC) istemporarily adsorbed to the first three way catalyst 7 and the secondthree way catalyst 8.

Then, as mentioned above in conjunction with FIG. 8, in the warming-upoperation of the internal combustion engine 1, the ECU 15 determines thelength of the fuel injection time for each cylinder so that the lengthof the fuel injection time of the first cylinder bank 1a is longer thanthe length of the fuel injection time of the second cylinder bank 1b. Inthis case, since the conditions other than that for the length of thefuel injection time are set the same for the first cylinder bank 1a andthe second cylinder bank 1b, the combustion of the leaner mixture ofeach cylinder of the second cylinder bank 1b than that of each cylinderof the first cylinder bank 1a is performed. As a result, the combustiongas temperature of each cylinder of the second cylinder bank 1b ishigher than the combustion gas temperature of each cylinder of the firstcylinder bank 1a.

Therefore, the temperature of the exhaust gas discharged from the secondcylinder bank 1b is higher than that of the exhaust gas discharged fromthe first cylinder bank 1a. However, since the second three way catalyst8 is provided at a position farther from the exhaust port, thetemperature change of the first three way catalyst 7 is substantiallysimilar to the temperature change of the second three way catalyst 8.

Namely, even if the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is high, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the air/fuel ratio of each cylinder.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same time and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon is introduced into the third three way catalyst 9 is short,it is possible to prevent the catalyst temperature of the third threeway catalyst 9 from dropping below the activated temperature.Accordingly, it is possible to purify all the unburnt hydrocarbon (HC)which has been desorbed from the first three way catalyst 7 and thesecond three way catalyst 8. Also, it is possible to suppress thecapacity increase of the third three way catalyst and the enlargement ofthe heater 12 and battery 14 to prevent the electric power from beingwasted.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a fourth embodiment of the present invention will now bedescribed with reference to the drawings. In this case, only structurewhich is different from the above-described embodiments will bedescribed.

FIG. 10 is a view showing a schematic structure of an internalcombustion engine 1 to which the exhaust gas purifying apparatus for aninternal combustion engine in accordance with this embodiment is appliedand an exhaust system thereof.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b.

Then, serge tanks 32a and 32b, which are independent of each other, areprovided in the first cylinder bank 1a and the second cylinder bank 1bof the internal combustion engine 1. A first intake pipe 36 is connectedto the serge tank 32a on the side of the first cylinder bank 1a and asecond intake pipe 37 is connected to the serge tank 32b on the side ofthe second cylinder bank 1b. The above-described first and second intakepipes 36 and 37 are merged together on the upstream side to form asingle intake pipe 31.

An air cleaner 40 is connected to an end portion on the upstream side ofthe above-described intake pipe 31. An air flow meter 30 for outputtingan electric signal in response to an air mass flowing through the intakepipe 31 is mounted on the intake pipe 31 downstream of this air cleaner40. A throttle vale 33 for opening/closing the air passage within theintake pipe 31 is mounted in the intake pipe 31 downstream of the airflow meter 30.

Two bypass pipes 38 and 39 are connected to the intake pipe 31 betweenthe above-described throttle valve 33 and the air flow meter 30. Onebypass pipe 38 of these two bypass pipes 38 and 39 is connected to thefirst intake pipe 36 through a first idle speed control valve (ISCV),and the other bypass pipe 39 is connected to a second pipe 37 through asecond idle speed control valve (ISCV).

When the internal combustion engine 1 is kept under an idle condition(substantially closed condition of the throttle valve 33), theabove-described first and second idle speed control valves 34 and 35 areclosed in accordance with the control signal of the ECU 15, and thefresh air flowing through the intake pipe 31 upstream of the throttlevalve 33 is fed to the first intake pipe 36 and the second intake pipe37.

Also, a crank angle sensor 21 for outputting an electric signal forevery 10° rotation of the crankshaft (not shown) and a water temperaturesensor 28 for detecting a cooling water temperature are mounted on theinternal combustion engine 1.

Subsequently, the ECU 15 is connected to various sensors (not shown) inaddition to an IG sensor 22, a ST sensor 23, the air/fuel sensors 25 and26, the oxygen sensor 24, the crank angle sensor 21, a water temperaturesensor 28 and the air flow meter 30 and calculates an electricapplication timing of the heater 12, a secondary air feeding amount, asecondary air feeding timing, a fuel injection amount, a fuel injectiontiming, an ignition timing, an opening degree of the first and secondidle speed control valves 34 and 35 or the like, to control the relay13, the air pump 11, the first and second idle speed control valves 34and 35 and the like in accordance with the signals from the respectivesensors.

For example, when the ECU 15 judges the idle condition of the internalcombustion engine 1 from the stop condition of the vehicle, thesubstantially closed condition of the throttle valve 33 or the like, theECU 15 calculates a target RPM from a loading condition of a compressorfor an air conditioner, an output signal from the water temperaturesensor 28 or the like, compares the outputted target RPM with the engineRPM calculated on the basis of the output signal from the crank anglesensor 21 and performs the feedback control of the first and second idlespeed control valves 34 and 35 so that the actual engine RPM isidentified with the target RPM.

Then, in the case where the temperature of the cooling water is low asin the case where the internal combustion engine 1 is started in a coldcondition, in order to accelerate the warming up of the internalcombustion engine, the ECU 15 performs a control so that an openingdegree of each of the first and second idle speed control valves 34 and35 is increased and the engine RPM is increased by 500 more rpm than inthe normal condition. Subsequently, the ECU 15 performs a control suchthat the opening degree of each of the first and second idle speedcontrol valves 34 and 35 is reduced in response to the elevation of thecooling water.

Furthermore, the ECU 15 in accordance with the embodiment performs sucha control that the opening degree of the second idle speed control valve35 is larger than the opening degree of the first idle speed controlvalve 34 and the intake air amount of each cylinder of the secondcylinder bank 1b is greater than the intake air amount of each cylinderof the first cylinder bank 1a. In this case, the exhaust gas amountdischarged from each of the cylinder of the second cylinder bank 1b isgreater than the exhaust gas amount discharged from each cylinder of thefirst cylinder bank 1a.

Thus, the ECU 15 and the first and second idle speed control valves 34and 35 realize a desorption/adjusting mechanism for adjusting the intakeair amount of the cylinders, to which each exhaust passage (firstexhaust pipe 4, second exhaust pipe 5) is connected, relative to eachother and adjusting the exhaust gas amounts discharged from everycylinder into the exhaust passage.

The operation and advantage of the exhaust gas purifying apparatus foran internal combustion engine in accordance with this embodiment willnow be described.

When the electric signal representative of the ON condition of theignition switch is inputted into the ECU 15 by the IG sensor 22 in thestarting operation for the internal combustion engine 1, the ECU 15switches the relay 13 from the OFF condition to the ON condition andapplies the current from the battery 14 to the heater 12 of the thirdthree way catalyst 9.

Subsequently, when an electric signal representative of the ON conditionof the starter switch is inputted into the ECU 15 by the ST sensor 23,the ECU 15 feeds a drive current to the air pump 11, pressurizes andfeeds the fresh air, flowing through the intake flow passage downstreamof the air cleaner 40, to the exhaust pipe 6 and causes the air/fuelratio of the exhaust gas introduced into the third three way catalyst 9to be close to the stoichiometric air/fuel ratio.

Subsequently, when the start of the internal combustion engine 1 iscompleted, the ECU 15 calculates a target RPM from a loading conditionof a compressor for an air conditioner, an output signal from the watertemperature sensor 28 or the like, compares the outputted target RPMwith the engine RPM calculated on the basis of the output signal fromthe crank angle sensor 21 and performs the feedback control of the firstand second idle speed control valves 34 and 35 so that the actual engineRPM is identified with the target RPM.

In this case, the ECU 15 controls the first and second idle speedcontrol valves 34 and 35 so that the opening degree of the second idlespeed control valve 35 on the side of the second cylinder bank 1b isgreater than the opening degree of the first idle speed control valve 34on the side of the first cylinder bank 1a.

As a result, since the intake air amount of each cylinder of the secondcylinder bank 1b is greater than the intake air amount of each cylinderof the first cylinder bank 1a, the amount of the exhaust gas dischargedfrom each cylinder of the second cylinder bank 1b is greater than theamount of the exhaust gas discharged from each cylinder of the firstcylinder bank 1a. This state is shown in FIG. 11. From FIG. 11, it isunderstood that after completion of the starting operation of theinternal combustion engine 1, the opening degree of the second idlespeed control valve 35 is greater than the opening degree of the firstidle speed control valve 34 so that the amount of the exhaust gasdischarged from each cylinder of the second cylinder bank 1b is greaterthan the amount of the exhaust gas discharged from each cylinder of thefirst cylinder bank 1a.

Thus, the exhaust gas discharged from the respective cylinder banks 1aand 1b of the internal combustion engine 1 are caused to flow into thefirst exhaust manifold 2 and the second exhaust manifold 3. The exhaustgas discharged from each cylinder of the first cylinder bank 1a iscaused to flow through the first exhaust manifold 2 into the firstexhaust pipe 4, and the exhaust gas discharged from each cylinder of thesecond cylinder bank 1b is caused to flow through the second exhaustmanifold 3 into the second exhaust pipe 5.

The exhaust gas that has been caused to flow into the first exhaust pipe4 flows into the first three way catalyst 7 midway along the firstexhaust pipe 4 and the unburnt hydrocarbon (HC) contained in the exhaustgas is temporarily adsorbed onto the first three way catalyst 7. Then,the exhaust gas discharged from the first three way catalyst 7 flowsinto the third three way catalyst 9 through the first exhaust pipe 4 andthe exhaust pipe 6.

On the other hand, the exhaust gas that has been caused to flow into thesecond exhaust pipe 5 flows into the second three way catalyst 8 midwayalong the second exhaust pipe 5 and the unburnt hydrocarbon (HC)contained in the exhaust gas is temporarily adsorbed onto the secondthree way catalyst 8. Then, the exhaust gas discharged from the secondthree way catalyst 8 flows into the third three way catalyst 9 throughthe second exhaust pipe 5 and the exhaust pipe 6.

Since the unburnt hydrocarbon (HC) has been removed from the exhaust gasfrom the first exhaust pipe 4 and the second exhaust pipe 5 by the firstthree way catalyst 7 and the second three way catalyst 8, even if thethird three way catalyst 9 is not activated, the unburnt hydrocarbon(HC) is not discharged downstream of the third three way catalyst 9.

Subsequently, the temperature of the first three way catalyst 7 and thesecond three way catalyst 8 is elevated by the heat of the exhaust gasbut the amount of the exhaust gas discharged from the second cylinderbank 1b is greater than the amount of the exhaust gas discharged fromthe first cylinder bank 1a. As a result, the temperature of the exhaustgas discharged from the second cylinder bank 1b is higher than thetemperature of the exhaust gas discharged from the first cylinder bank1a.

However, since the second three way catalyst 8 is provided at a positionfarther from the exhaust port, the temperature change of the first threeway catalyst 7 is substantially similar to the temperature change of thesecond three way catalyst 8.

Namely, even if the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is high, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the amount of the intake air of eachcylinder.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same period and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon introduces into the third three way catalyst 9 is short, itis possible to prevent the catalyst temperature of the third three waycatalyst 9 from dropping below the activated temperature. Accordingly,it is possible to purify all the unburnt hydrocarbon (HC) which has beendesorbed from the first three way catalyst 7 and the second three waycatalyst 8. Also, it is possible to suppress the capacity increase ofthe third three way catalyst 9 and suppress enlargement of the heater 12and battery 14 to prevent the electric power from being wasted.

Incidentally, as a method for adjusting the intake air amount of thefirst cylinder bank 1a from the intake air amount of the second cylinderbank 1b, it is possible to provide sub-throttle valves which are drivenby a motor instead of the first and second idle speed control valves 34and 35 in the first intake pipe 36 and the second intake pipe 37.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a fifth embodiment of the present invention will now bedescribed with reference to the drawings. In this case, only structurewhich is different from the above-described embodiment will bedescribed.

FIG. 12 is a view showing a schematic structure of an internalcombustion engine 1 to which the exhaust gas purifying apparatus for aninternal combustion engine in accordance with this embodiment is appliedand an exhaust system thereof. In this embodiment, the distance from thefirst three way catalyst 7 to the exhaust port of the first cylinderbank 1a is shorter than the distance from the second three way catalyst8 to the exhaust port of the second cylinder bank 1b.

A first cylinder bank side serge tank 32a is connected to an intake portof each cylinder of the first cylinder bank 1a through an intakemanifold 41a. An intake flow control valve 42a for opening/closing aflow path within the intake manifold 41a is mounted in each branch pipeof the intake manifold 41a. An air assist nozzle 43a for injecting freshair, flowing downstream of the air cleaner 40, into the intake manifold41a is mounted in the intake manifold 41a downstream of the intake flowcontrol valve 42a.

The above-described intake flow control valve 42a may be switched overamong a fully open condition, a half-open condition and fully closedcondition by an actuator 46a. The actuator 46a switches the open/closedconditions of the above-described intake flow control valve 42a inresponse to a control signal from the ECU 15.

Each air assist nozzle 43a on the side of the first cylinder bank 1a isconnected to an idle speed control valve 34 through an air delivery pipe44. In this case, the idle speed control valve 34 in accordance withthis embodiment is formed by a three-way valve for switching the flowpaths so that the fresh air introduced from the intake pipe 31 upstreamof the throttle valve 33 is caused to flow into either air delivery pipe44 or first intake pipe 36.

Subsequently, a second serge tank 32b on the second cylinder bank sideis connected to an intake port of each cylinder of the second cylinderbank 1b through an intake manifold 41b. Then, in the same way as in thefirst cylinder bank side, an intake flow control valve 42b and an airassist nozzle 43b are mounted in each branch pipe of the intake manifold41b. The above-described intake flow control valve 42b is switched overamong a fully open condition, a half-open condition and fully closedcondition by an actuator 46b.

Then, each air assist nozzle 43b on the side of the second cylinder bank1b is connected to an idle speed control valve 35 through an airdelivery pipe 45. In the same manner as in the above-described idlespeed control valve 34, the idle speed control valve 35 is also formedby a three-way valve for switching the flow paths so that the fresh airintroduced from the intake pipe 31 upstream of the throttle valve 33 iscaused to flow into either air delivery pipe 45 or second intake pipe37. Subsequently, the ECU 15 is connected to various sensors (not shown)in addition to an IG sensor 22, a ST sensor 23, the air/fuel sensors 25and 26, the oxygen sensor 24, the crank angle sensor 21, a watertemperature sensor 28 and the air flow meter 30 and calculates anelectric application timing of the heater 12, a secondary air feedingamount, a secondary air feeding timing, a fuel injection amount, a fuelinjection timing, an ignition timing, an opening degree and a flow pathof the first and second idle speed control valves 34 and 35, an openingdegree of the intake flow control valves of each cylinder banks 1a, 1bor the like, to control the relay 13, the air pump 11, the first andsecond idle speed control valves 34 and 35, the actuators 46a and 46band the like in accordance with the signals from the respective sensors.

For example, in the starting operation of the internal combustion engine1, the ECU 15 feeds control signals to the actuators 46a and 46b, bringsall the intake control valves 42a and 42b of the first cylinder bank 1aand the second cylinder bank 1b into the fully closed condition, and atthe same time switches the flow paths of the idle speed control valves34 and 35 to the air delivery pipes 44 and 45.

Under such a condition, when the starting operation of the internalcombustion engine 1 (that is, the cranking is started) and the intakevalve of each cylinder is opened, the vacuum pressure is generateddownstream of the intake flow control valves 42a and 42b by the downwardmovement of the piston within each cylinder. In this case, fresh air isinjected from the air assist nozzles 43a and 43b, the fresh air isstrongly drawn into the combustion engine of each cylinder. As a resultof the high speed fresh air flow, the fuel injected from the injectionvalve (not shown) is strongly diffused to thereby produce a mist-likegood mixture.

Furthermore, the ECU 15 in accordance with this embodiment controls theactuators 46a and 46b so that the intake air flow control valves 42a onthe side of the first cylinder bank 1a are kept in the half-opencondition in the warming-up operation after the completion of thestarting operation of the internal combustion engine 1 whereas theintake flow control valves 42b on the side of the second cylinder bank1b are kept under the fully closed condition. In this case, the freshair that flows within the intake manifold 41a on the side of the firstcylinder bank 1a interferes with the intake flow control valves 42a keptunder the half-open condition to produce turbulence and flow into thecombustion chambers. Then, in each cylinder of the first cylinder bank1a, since the combustion speed is enhanced by the above-describedturbulence, the time period for combustion is shortened in comparisonwith each cylinder of the second cylinder bank 1b.

As a result, a time which is taken from the completion of the combustionin each cylinder of the first cylinder bank 1a until the exhaust valveis opened is longer than a time which is taken from the completion ofthe combustion in each cylinder of the second cylinder bank 1b until theexhaust valve is opened. Corresponding to this, the temperature of thecombustion gas within the combustion chamber is lowered. The temperatureof the exhaust gas from each cylinder of the first cylinder bank 1a islower than the temperature of the exhaust gas from each cylinder of thesecond cylinder bank 1b.

The operation and advantage of the exhaust gas purifying apparatus foran internal combustion engine in accordance with the embodiment will nowbe described.

When the electric signal representative of the ON condition of theignition switch is inputted into the ECU 15 by the IG sensor 22 in thestarting operation for the internal combustion engine 1, the ECU 15switches the relay 13 from the OFF condition to the ON condition andapplies the current from the battery 14 to the heater 12 of the thirdthree way catalyst 9. Then, the ECU 15 feeds the control signals to theactuators 46a and 46b, brings all the intake flow control valves 42a and42b of the first cylinder bank 1a and the second cylinder bank 1b intothe fully closed condition, and at the same time switches the flow pathsof the idle speed control valves 34 and 35 to the side of the airdelivery pipes 44 and 45. Subsequently, when an electric signalrepresentative of the ON condition of the starter switch is inputtedinto the ECU 15 by the ST sensor 23, the ECU 15 feeds a drive current tothe air pump 11, pressurizes and feeds the fresh air, flowing throughthe intake flow passage downstream of the air cleaner, to the exhaustpipe 6 and causes the air/fuel ratio of the exhaust gas introduced intothe third three way catalyst 9 to be close to the stoichiometricair/fuel ratio.

Subsequently, when the starting operation of the internal combustionengine 1 is completed, the ECU 15 switches the flow paths of the idlespeed control valves 34 and 35 from the side of the air delivery pipes44 and 45 to the side of the first intake pipe 36 and the second intakepipe 37. Then, the ECU 15 calculates a target RPM from a loadingcondition of a compressor, for an air conditioner, and an output signalfrom the water temperature sensor 28 or the like, and calculates thetarget RPM of the internal combustion engine 1 on the basis of theoutput signal from the crank angle sensor 21.

Subsequently, the ECU 15 compares the outputted target RPM with theengine RPM and performs the feedback control of the first and secondidle speed control valves 34 and 35 so that the actual engine RPM isidentified with the target RPM. In this case, the ECU 15 controls thefirst and second idle speed control valves 34 and 35 so that the openingdegree of the first idle speed control valve 34 on the side of the firstcylinder bank 1a is equal to the opening degree of the second idle speedcontrol valve 35 on the side of the second cylinder bank 1b.

Furthermore, the ECU 15 controls the actuator 46a so that the intakeflow control valves 42a on the side of the first cylinder bank 1a arekept under the half-open condition, and at the same time, controls theactuator 46b so that the intake flow control valves 42b on the side ofthe second cylinder bank 1b are kept under the half-open condition.

In this case, the fresh air flowing trough the intake manifold 41a onthe side of the first cylinder bank 1a is introduced into the combustionchambers while interfering with the intake flow control valves 42a keptunder the half-open condition to produce the turbulence, whereas thefresh air flowing through the intake manifold 41b on the second cylinderbank 1b hardly interferes with the intake flow control valves 42b keptunder the fully closed condition and is introduced into the combustionchambers without any turbulence. For this reason, the combustion speedof each cylinder of the first cylinder bank 1a is higher than thecombustion speed of each cylinder of the second cylinder bank 1b.

Then, in the opening state of the exhaust valves, the combustion gastemperature within each cylinder of the first cylinder bank 1a is lowerthan the combustion gas temperature within each cylinder of the secondcylinder bank 1b. The temperature of the exhaust gas discharged fromeach cylinder of the first cylinder bank 1a is lower than the eachcylinder of the second cylinder bank 1b.

However, since the second three way catalyst 8 is provided at a positionfarther from the exhaust port, the temperature change of the first threeway catalyst 7 is substantially similar to the temperature change of thesecond three way catalyst 8.

Namely, even if the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is high, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the intake flow or a degree of themixture which is introduced into the combustion chambers of eachcylinder.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same period and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon introduces into the third three way catalyst 9 is short, itis possible to prevent the catalyst temperature of the third three waycatalyst 9 from dropping below the activated temperature. Accordingly,it is possible to purify all the unburnt hydrocarbon (HC) which has beendesorbed from the first three way catalyst 7 and the second three waycatalyst 8. Also, it is possible to suppress the capacity increase ofthe third three way catalyst and the enlargement of the heater 12 andbattery 14 to prevent the electric power from being wasted.

Incidentally, there is described an example in which the intake flowcontrol valves 42a and 42b control the intake flow which is introducedinto the combustion chambers of each cylinder. However, needless to say,it is possible to control the air assist instead of the intake flow.

Also, the direction of the intake flow is selected from at least one ofvertical swirl and horizontal swirl. Furthermore, in case of a directinjection combustion engine having an injection valve in each cylinder,it is possible to change the degree of mixture on the basis of fuelinjection timing.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a sixth embodiment of the present invention will now bedescribed with reference to the drawings. In this case, only structurewhich is different from the above-described embodiment will bedescribed.

FIG. 13 is a view showing a schematic structure of an internalcombustion engine 1 to which the exhaust gas purifying apparatus for aninternal combustion engine in accordance with this embodiment is appliedand an exhaust system thereof.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b.

Each cylinder of the first cylinder bank 1a of the internal combustionengine 1 is provided with a straight port 60 having a straight flow pathfrom an opening portion formed in an outer wall of a cylinder headtoward an opening portion formed in the combustion chamber and a helicalport 47 having a flow path swirled from an opening portion of the outerwall of the cylinder head toward an opening portion formed in thecombustion chamber.

The straight port 60 of each cylinder of the first cylinder bank 1a isconnected to a serge tank 32 through a first straight port side intakemanifold 48, and the helical port 47 of each cylinder is connected tothe serge tank 32 through the first helical port side intake manifold49. Then, a first swirl control valve 50 for opening/closing a flow pathwithin the pipe is provided in the first straight port side intakemanifold 48 and is driven by an actuator 51.

On the other hand, in the same way as for the first cylinder bank 1a,each cylinder of the second cylinder bank 1b of the internal combustionengine is provided with a straight port 53 and a helical port 54. Thestraight port 53 is connected to the serge tank 32 through a secondstraight port side intake manifold 56, and the helical port 54 isconnected to the serge tank 32 through the second helical port sideintake manifold 57. Then, a second swirl control valve 55 is provided inthe first straight port side intake manifold 56 and is driven by anactuator 52.

Subsequently, the ECU 15 is connected to various sensors such as thecrank angle sensor 21, and a water temperature sensor 28 or the like andcalculates an electric application timing of the heater 12, a secondaryair feeding amount, a secondary air feeding timing, a fuel injectionamount, a fuel injection timing, an ignition timing, an opening degreeof the first and second swirl control valves 55 and 56 or the like, tocontrol the relay 13, the air pump 11, the actuators 51 and 52 or thelike in accordance with the signals from the respective sensors.

Then, during the warming-up operation after the completion of thestarting operation of the internal combustion engine 1, the ECU 15 inaccordance with this embodiment controls the actuators 51 and 52 so thatthe first swirl control valve 50 on the side of the first cylinder bank1a is closed and the second swirl control valve 55 on the side of thesecond cylinder bank 1b is opened.

In this case, the fresh air or mixture is introduced only from thehelical port 47 into the combustion chamber of each cylinder of thefirst cylinder bank 1a to generate a strong swirl flow within thecombustion chamber. The flame propagation of each cylinder of the firstcylinder bank 1a is accelerated by the swirl flow to enhance thecombustion speed.

On the other hand, the fresh air or mixture is introduced from both thehelical portion 54 and the straight port 53 into the combustion chamberof each cylinder of the second cylinder bank 1b so that a strong swirlis not generated in the combustion chamber. As a result, in eachcylinder of the second cylinder bank 1b, the flame propagation is notaccelerated like the first cylinder bank 1a. Accordingly, the combustionspeed of each cylinder of the second cylinder bank 1b is slower thanthat of each cylinder of the first cylinder bank 1a.

Accordingly, in the opening state of the exhaust valves, the combustiongas temperature within each cylinder of the first cylinder bank 1a islower than the combustion gas temperature within each cylinder of thesecond cylinder bank 1b. The temperature of the exhaust gas dischargedfrom each cylinder of the second cylinder bank 1b is higher than theeach cylinder of the first cylinder bank 1a.

However, since the second three way catalyst 8 is provided at a positionfarther from the exhaust port, the temperature change of the first threeway catalyst 7 is substantially similar to the temperature change of thesecond three way catalyst 8.

Namely, even if the high temperature of the exhaust gas is dischargedfrom the second cylinder bank 1b, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the intake flow which is introducedinto the combustion chambers of each cylinder.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same time and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon introduces into the third three way catalyst 9 is short, itis possible to prevent the catalyst temperature of the third three waycatalyst 9 from dropping below the activated temperature. Accordingly,it is possible to purify all the unburnt hydrocarbon (HC) which has beendesorbed from the first three way catalyst 7 and the second three waycatalyst 8. Also, it is possible to suppress the capacity increase ofthe third three way catalyst and the enlargement of the heater 12 andbattery 14 to prevent the electric power from being wasted.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a seventh embodiment of the present invention will nowbe described with reference to the drawings. In this case, onlystructure which is different from the above-described embodiment will bedescribed.

FIG. 14 is a view showing a schematic structure of an internalcombustion engine 1 to which the exhaust gas purifying apparatus for aninternal combustion engine in accordance with this embodiment is appliedand an exhaust system thereof.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b.

Then, each cylinder of the first cylinder bank 1a of the internalcombustion engine 1 is provided with a first variable valve timingmechanism 58 for changing a rotational phase of a cam shaft (not shown)for driving an intake valve of each cylinder, and each cylinder of thesecond cylinder bank 1b of the internal combustion engine 1 is providedwith a second variable valve timing mechanism 59 for changing arotational phase of the cam shaft (not shown) for driving an intakevalve of each cylinder. These first and second valve timing mechanisms58 and 59 change the phases of the cam shafts in accordance with controlsignals from the ECU 15.

Subsequently, a cam position sensor 20a for detecting a rotationalposition of the cam shaft on the exhaust valve side is mounted on thefirst cylinder bank 1a, and a cam position sensor 20b for detecting arotational position of the cam shaft on the exhaust valve side ismounted on the second cylinder bank 1b.

Next, the ECU 15 calculates an optimum opening/closing timing (targetvalve timing) for each intake valve in response to the operationalconditions such as an engine RPM, an intake air amount or the like ofthe internal combustion engine 1, simultaneously calculates an actualopening/closing timing (actual valve timing) in accordance with outputsignals of the cam position sensors 20a and 20b and controls the firstand second variable valve timing mechanisms 58 and 59 so that the actualvalve timing is identified with the target valve timing.

Furthermore, the ECU 15 in accordance with this embodiment controls thefirst and second variable valve timing mechanisms 58 and 59 so that theopening timing of the intake valve of the second cylinder bank 1b isearlier than the opening timing of the intake valve of the firstcylinder bank 1a in the warming-up operation after the completion of thestarting operation of the internal combustion engine 1. In this case, inthe opening state of the intake valves of the second cylinder bank 1b,the combustion gas temperature within each cylinder of the secondcylinder bank 1b is higher than the combustion gas temperature withineach cylinder of the first cylinder bank 1a when the exhaust valve ofthe first cylinder bank 1a. Accordingly, the temperature of the exhaustgas discharged from each cylinder of the second cylinder bank 1b ishigher than the each cylinder of the first cylinder bank 1a.

However, since the second three way catalyst 8 is provided at a positionfarther from the exhaust port, the temperature change of the first threeway catalyst 7 is substantially similar to the temperature change of thesecond three way catalyst 8.

Namely, even if the temperature of the exhaust gas discharged from thesecond cylinder bank 1b is high, the temperature of the exhaust gasthrough the second exhaust pipe 5 is dropped to the temperature of thefirst three way catalyst 7 by radiating heat from the exhaust passagewhen the exhaust gas reaches to the second three way catalyst 8.Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is controlled by adjusting the opening timing of the intakevalve and the exhaust valve of each cylinder.

Accordingly, in accordance with this embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon is introduced into the third three way catalyst 9 is short,it is possible to prevent the catalyst temperature of the third threeway catalyst 9 from dropping below the activated temperature.Accordingly, it is possible to purify all the unburnt hydrocarbon (HC)which has been desorbed from the first three way catalyst 7 and thesecond three way catalyst 8. Also, it is possible to suppress thecapacity increase of the third three way catalyst and the enlargement ofthe heater 12 and battery 14 to prevent the electric power from beingwasted.

Incidentally, there is described an example in which the first andsecond variable valve timing mechanisms control the opening timing ofthe intake valves of each cylinder. However, needless to say, it ispossible to control the opening timing of the exhaust valves. Also, itis possible to control the opening timing of both the intake valves andthe exhaust valves.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with an eighth embodiment of the present invention will nowbe described. In this case, only structure which is different from theabove-described embodiment will be described.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b. Also, the conditions of the exhaust gaspurifying apparatus are set at the same conditions except the structureof the exhaust manifolds and the exhaust pipes. Accordingly, thetemperature of the exhaust gas which is discharged from each cylinder ofthe first cylinder bank 1a is equal to the temperature of the exhaustgas which is discharged from each cylinder of the second cylinder bank1b.

Then, the first exhaust manifold 2 is made of stainless steel and thesecond exhaust manifold 3 is made of cast iron. In this case, since thestainless steel has a thermal capacity which is higher than that of thecast iron, the thermal capacity of the first exhaust manifold 2 isgreater than that of the second exhaust manifold 3.

The thus constructed first and second exhaust manifolds 2 and 3 adsorbthe heat of the exhaust gas discharged from the internal combustionengine 1 but the thermal capacity of the first exhaust manifold 2 isgreater than that of the second exhaust manifold 3. Accordingly, thefirst exhaust manifold 2 deprives more heat from the exhaust gas thanthe second exhaust manifold 3. Thus, the temperature of the exhaust gasflowing through the first exhaust manifold 2 is lower than thetemperature of the exhaust gas flowing through the second exhaustmanifold 3.

As a result, the temperature of the exhaust gas which is introduced intothe first three way catalyst 7 is dropped to the temperature of theexhaust gas which is introduced into the second three way catalyst 8,and the temperature change of the first three way catalyst 7 issubstantially similar to the temperature change of the second three waycatalyst 8.

Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of the exhaust gas which is introduced into each three waycatalyst is adjusted by the thermal capacity of the exhaust manifolds.

Thus, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same period and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon is introduced into the third three way catalyst 9 is short,it is possible to prevent the catalyst temperature of the third threeway catalyst 9 from dropping below the activated temperature.Accordingly, it is possible to purify all the unburnt hydrocarbon (HC)which has been desorbed from the first three way catalyst 7 and thesecond three way catalyst 8. And, it is possible to suppress thecapacity increase of the third three way catalyst and the enlargement ofthe heater 12 and battery 14 to prevent the electric power from beingwasted.

Thus, the first exhaust manifold 2 and the second exhaust manifold 3realize a desorption/adjustment mechanism in accordance with the presentinvention.

Incidentally, there is described an example in which the first andsecond exhaust manifolds are made of stainless steel and cast iron.However, needless to say, it is possible for the exhaust manifolds to bemade of other material on the basis of the distance from the exhaustport to each three way catalyst, the temperature of exhaust gas in thewarming-up operation and the amount of the exhaust gas.

An exhaust gas purifying apparatus for an internal combustion engine inaccordance with a ninth embodiment of the present invention will now bedescribed. In this case, only structure which is different from theabove-described embodiment will be described.

In this embodiment, the distance from the first three way catalyst 7 tothe exhaust port of the first cylinder bank 1a is shorter than thedistance from the second three way catalyst 8 to the exhaust port of thesecond cylinder bank 1b. And, the conditions except the structure of theexhaust manifolds and the exhaust pipes are set at the same conditions.Accordingly, the temperature of the exhaust gas which is discharged fromeach cylinder of the first cylinder bank 1a is equal to the temperatureof the exhaust gas which is discharged from each cylinder of the secondcylinder bank 1b.

The first three way catalyst 7 and the second three way catalyst 8 areformed by filling cylindrical outer sleeves with monolithic catalystshaving through holes in the flow direction of the exhaust gas. In thiscase, the number of through holes per unit area of the first three waycatalyst 7 is greater than the number of the through holes per unit areaof the second three way catalyst 8.

In this case, since the density of the through holes per unit area ofthe first three way catalyst 7 is greater than the density of thethrough holes per unit area of the second three way catalyst 8, the heatcapacity of the first three way catalyst 7 is greater than the heatcapacity of the second three way catalyst 8. Thus, the above-describedfirst and second three way catalysts 7 and 8 realize thedesorption/adjustment mechanism according to the present invention.

Then, when the internal combustion engine 1 is started, theabove-described first and second three way catalysts 7 and 8 receive theheat of the exhaust gas discharged from the internal combustion engine 1with their temperature being elevated. However, since the heat capacityof the second three way catalyst 8 is smaller than the heat capacity ofthe first three way catalyst 7, the temperature elevation rate of thesecond three way catalyst 8 is higher than the temperature elevationrate of the first three way catalyst 7.

A relationship between the heat capacity of the three way catalyst andthe desorption timing will now be described with reference to FIG. 15. Acurve "j" in FIG. 15 shows a result of the measurement of the HCconcentration in the exhaust gas upstream of the first three waycatalyst 7 or the second three way catalyst 8. It is understood that alarge amount of HC exists in the exhaust gas in the starting operationof the internal combustion engine 1.

Subsequently, curves "k" in FIG. 15 show results of the measurement ofthe HC concentration in the exhaust gas downstream of the three waycatalysts having four different capacities. It is understood that thegreater the heat capacity of the three way catalyst, the slower thedesorption timing of the unburnt hydrocarbon (HC) will become.

Thus, the three way catalysts which have different heat capacities areutilized so that the timings of desorption of the unburnt hydrocarbon(HC) by the respective three way catalysts may be synchronized with eachother.

Accordingly, in this embodiment, in order to correct the temperaturechange of the exhaust gas which is caused by the difference of thedistance from each three way catalyst to the exhaust port, thetemperature of each three way catalyst is adjusted by its heat capacity.Then, the first and second three way catalysts 7 and 8 reach thepredetermined temperature at the same period and desorb the unburnthydrocarbon (HC) adsorbed in the starting operation of the internalcombustion engine 1. Then, the unburnt hydrocarbon (HC) desorbed fromthe each three way catalyst is discharged from each three way catalysttogether with the exhaust gas and introduced into the third three waycatalyst 9 through the exhaust pipe 6. Here, since the exhaust gassystem of the internal combustion engine 1 is designed so that the thirdthree way catalyst 9 is activated by a heater 12 before the temperatureof the first and second three way catalysts 7 and 8 reach the desorptiontemperature, the unburnt hydrocarbon (HC) desorbed from each three waycatalyst is oxidized or reduced by the third three way catalyst 9 in theshort time.

Accordingly, in accordance with the embodiment, it is possible tosynchronize the timing for desorbing the unburnt hydrocarbon (HC) by thefirst three way catalyst 7 with the timing for desorbing the unburnthydrocarbon (HC) by the second three way catalyst 8 and to introduce allthe unburnt hydrocarbon (HC) adsorbed to the first three way catalyst 7and the second three way catalyst 8 into the third three way catalyst 9at the same time. As a result, since the time when the unburnthydrocarbon is introduced into the third three way catalyst 9 is short,it is possible to prevent the catalyst temperature of the third threeway catalyst 9 from dropping below the activated temperature.Accordingly, it is possible to purify all the unburnt hydrocarbon (HC)which has been desorbed from the first three way catalyst 7 and thesecond three way catalyst 8. Also, it is possible to suppress thecapacity increase of the third three way catalyst and the enlargement ofthe heater 12 and battery 14 to prevent the electric power from beingwasted.

Incidentally, as a method for adjusting the heat capacities of the firstthree way catalyst 7 and the second three way catalyst 8, it is possibleto differentiate a thickness of a ceramic carrier constituting the firstthree way catalyst 7 from a thickness of a ceramic carrier constitutingthe second three way catalyst 8. For example, in the case where thethickness of the ceramic carrier of the first three way catalyst 7 isgreater than the thickness of the ceramic carrier of the second threeway catalyst 8, the heat capacity of the first three way catalyst 7 isgreater than the heat capacity of the second three way catalyst 8.

Also, it is possible to adjust a thickness of an alumina coatconstituting a catalyst layer of the first three way catalyst 7 from athickness of an alumina coat constituting a catalyst layer of the secondthree way catalyst 8. For example, in the case where the thickness ofthe alumina coat of the first three way catalyst 7 is greater than thethickness of the alumina coat of the second three way catalyst 8, theheat capacity of the first three way catalyst 7 is greater than the heatcapacity of the second three way catalyst 8.

Furthermore, it is possible to adjust an amount of a catalyst substancecarried on an alumina coat of the first three way catalyst 7 from anamount of a catalyst substance carried on an alumina coat of the secondthree way catalyst 8. For example, in the case where the heat capacityof the catalyst substance of the first three way catalyst 7 is greaterthan the heat capacity of the catalyst substance of the second three waycatalyst 8, the heat capacity of the first three way catalyst 7 isgreater than the heat capacity of the second three way catalyst 8.

Also, it is possible to adjust a capacity of the first three waycatalyst 7 from a capacity Of the second three way catalyst 8. Forexample, in the case where the capacity of the first three way catalyst7 is greater than the capacity of the second three way catalyst 8, theheat capacity of the first three way catalyst 7 is greater than the heatcapacity of the second three way catalyst 8.

Furthermore, it is possible to form the carrier of the first three waycatalyst 7 and the carrier of the second three way catalyst 8 fromdifferent materials. For example, in the case where the carrier of thefirst three way catalyst 7 is made of metal and the carrier of thesecond three way catalyst 8 is made of ceramic, since the heat capacityof the metal is greater than the capacity of the ceramic, the heatcapacity of the first three way catalyst 7 is greater than the heatcapacity of the second three way catalyst 8.

Also, it is possible to adjust a thickness of an outer sleeveconstituting the first three way catalyst 7 from a thickness of an outersleeve constituting the second three way catalyst 8. For example, in thecase where the thickness of the outer sleeve of the first three waycatalyst 7 is greater than the thickness of the outer sleeve of thesecond three way catalyst 8, the heat capacity of the first three waycatalyst 7 is greater than the heat capacity of the second three waycatalyst 8.

Subsequently, it is possible to exemplify a method for adjusting heattransfer properties of both of the first three way catalyst 7 and thesecond three way catalyst 8 as a method for synchronizing the desorptiontimings of the first three way catalyst 7 and the second three waycatalyst 8. For example, in the case where the capacities of the firstthree way catalyst 7 and the second three way catalyst 8 are kept at thesame level and the second three way catalyst 8 is thicker and shorterthan the first three way catalyst 7, it is more difficult to transferthe heat of the end portion on the downstream side in the first threeway catalyst 7 than in the second three way catalyst 8, and it takes along time to elevate the temperature up to the predeterminedtemperature. As a result, the second three way catalyst 8 reaches thepredetermined temperature earlier than the first three way catalyst 7.

In the foregoing first to ninth embodiments, the exhaust gas purifyingapparatus for an internal combustion engine according to the presentinvention is applied to a V-shaped multi-cylinder internal combustionengine. However, it may be applied to a straight type multi-cylinderinternal combustion engine. For example, as shown in FIG. 16, in case ofthe straight internal combustion engine having six cylinders, a firstexhaust manifold 2a is connected to first through third cylinders and asecond exhaust manifold 2b is connected to fourth through sixthcylinders. Subsequently, in a dual structure exhaust pipe in which thefirst pipe 4 is connected to the above-described first exhaust manifold2a, and the second exhaust pipe 5 is connected to the above-describedexhaust manifold 2b, it is possible to synchronize a timing fordesorbing the unburnt hydrocarbon (HC) by the first three way catalyst 7provided in the above-described first exhaust pipe 4 with the timing fordesorbing the unburnt hydrocarbon (HC) by the second three way catalyst8 provided in the above-described second exhaust pipe 5. Also, it ispossible to synchronize a timing for introducing into the third threeway catalyst 9 with a timing for introducing into the third three waycatalyst 9.

Subsequently, it is possible to exemplify a method for adjusting theamount of the exhaust gas or the velocity of a exhaust gas downstream ofthe first three way catalyst 7 and the second three way catalyst 8. Forexample, it is possible to provide an exhaust control valve between thefirst three way catalyst 7 and the third three way catalyst 9 in theexhaust pipe.

Also, it is possible to exemplify a method for adjusting a length of thefirst exhaust pipe 4 and a length of the second exhaust pipe 5.

Incidentally, there is described an example in which the first andsecond three way catalysts correspond to adsorption/desorptionmechanisms according to the present invention. However, needless to say,it is possible for the adsorption/desorption mechanisms to be made ofzeolite or activated carbon.

From the invention thus described, it will be obvious that the inventionmay be varied in many ways. Such variations are intended to be withinthe spirit and scope of the invention, and all such modification obviousto one of ordinary skill in the art are intended to be within the scopeof the following claims.

What is claimed is:
 1. An exhaust gas purifying apparatus for an internal combustion engine, comprising:a plurality of exhaust passages connected to a multi-cylinder internal combustion engine; a joint exhaust passage formed by merging said plurality of exhaust passages; an exhaust gas purifier that purifies exhaust gas that flows through said joint exhaust passage; an adsorption/desorption unit provided in each of said exhaust passages that adsorbs an unburnt gas component contained in the exhaust gas that flows through each of said exhaust passages at a temperature lower than a predetermined temperature and desorbs the adsorbed unburnt gas component at a temperature equal to or higher than the predetermined temperature; and a desorption/adjustment mechanism that synchronizes the timing of each of said adsorption/desorption units to desorb the unburnt gas components therefrom so that the introduction timing of the unburnt gas components, which have been desorbed from the respective adsorption/desorption units, into the exhaust gas purifier occurs at the same time.
 2. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said desorption/adjustment mechanism independently controls a temperature of the exhaust gas introduced into said adsorption/desorption unit of each of said exhaust passages to achieve the synchronized timing of the desorbed unburnt gas components into the exhaust gas purifier.
 3. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein said desorption/adjustment mechanism controls exhaust temperature on the basic of distances between each said adsorption/desorption unit and the internal combustion engine.
 4. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein said desorption/adjustment mechanism adjusts ignition timing of cylinders to which the respective exhaust passages are connected to control temperatures of the exhaust gas flowing through the respective exhaust passages from the cylinders.
 5. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein said desorption/adjustment mechanism adjusts air/fuel mixture ratios to be burnt in the cylinders to which the respective exhaust passages are connected to control temperatures of the exhaust gas flowing through the respective exhaust passages from the cylinders.
 6. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein said desorption/adjustment mechanism adjusts intake air amounts of the cylinders to which the respective exhaust passage are connected to adjust exhaust amounts of the exhaust gas discharged from the cylinders for every exhaust passage.
 7. The exhaust gas purifying apparatus for an internal combustion engine according to claim 2, wherein said desorption/adjustment mechanism adjusts heat capacities of the respective exhaust passages.
 8. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said desorption/adjustment mechanism individually adjusts heat capacities of the respective adsorption/desorption units to achieve the synchronized timing.
 9. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a thickness of a member constituting said carrier for each of said adsorption/desorption units.
 10. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a thickness of a member constituting said outer sleeve for each said adsorption/desorption unit.
 11. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a density of said through holes for each said adsorption/desorption unit.
 12. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a diameter of said carrier for each said adsorption/desorption unit.
 13. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a length in an axial direction of said carrier for each of said adsorption/desorption units.
 14. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a volume of said carrier for each of said adsorption/desorption units.
 15. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts a material of a member constituting said carrier for each of said adsorption/desorption units.
 16. The exhaust gas purifying apparatus for an internal combustion engine according to claim 8, wherein each said adsorption/desorption unit includes a carrier having a plurality of through holes in a direction of the flow of the exhaust gas, a catalyst layer formed on a surface of said carrier and an outer sleeve on which said carrier is provided; and wherein said desorption/adjustment mechanism adjusts an amount of the catalyst material for each of said adsorption/desorption units.
 17. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said internal combustion engine is a V-shaped internal combustion engine having a first cylinder bank and a second cylinder bank in which two or more cylinders are arranged in a straight line and said exhaust passages are exhaust pipes connected to the respective cylinders banks.
 18. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said exhaust passages are a dual exhaust pipe connected to the internal combustion engine.
 19. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said adsorption/desorption units include a three-way catalyst.
 20. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said adsorption/desorption units include an adsorbent including a zeolite.
 21. The exhaust gas purifying apparatus for an internal combustion engine according to claim 1, wherein said exhaust gas purifier includes a catalyst provided with a heater.
 22. The exhaust gas purifying apparatus for an internal combustion engine according to claim 21, further comprising heater control means for feeding a current to the heater to activate the catalyst before the unburnt gas components desorbed from said adsorption/desorption units are introduced into said exhaust gas purifier. 